Patellofemoral joint pain (PFP) and its sequelae Patellofemoral joint osteoarthritis have been reported to occur in significant numbers of patients following ACL reconstruction (ACLR) and have a substantial impact on function and quality of life1. Researchers have for a long time now theorized that a diminished capacity of the quadriceps to control loads and provide joint stability may place abnormal stresses on articular structures of the knee, predisposing the knee to degeneration2,3. The first part of this blog will present some of the growing body of evidence showing that ACLR patients have deficits in performance which can be linked to deficiencies in their quadriceps function, both of which can then be associated with abnormal patellofemoral joint loading. The second part of the blog will discuss how ACLR rehab might have to be modified to reduce the ACLR patient’s susceptibility to developing PFP.

ACLR patients have abnormal movement patterns such as decreased knee flexion angles (and excursion), knee extensor moments and higher initial peak loading rates compared to controls, when descending stairs, landing, walking and running, which are present from three months post op and can persist beyond two years post op4,5,6,7. These behaviours are indicative of “quads avoidance” a phenomena described by Powers et al8 as occurring in PFP patients. Alongside the abnormal movement patterns ACLR patients also present with significant deficits in quadriceps strength, force development and activation levels (degree of inhibition)6,9,10,11. These strength changes and movement patterns would appear to pre date the onset of osteoarthritis certainly and possibly PFP reported by Culvenor1. So the question might be why could these changes predispose the individual to PFP?

Why ACLR patients develop PFP could be due to a number of factors, but principle amongst them is likely to be the development and maintenance of bony oedema and the alteration in the loading environment within the patellofemoral joint. The presence and extent of subchondral bone marrow lesion (bony oedema) is significantly related to subchondral bone attrition and damage12, with up to 80% of ACLR patients having significant bony oedema on MRI scanning, so provides an overt link to osteoarthritis. Elevated levels of water content in the patella have been shown to be related to increased pain levels in PFP patients13, linking this to PFP. The presence of pain and bony oedema is likely to have a significant impact on levels of quadriceps inhibition and so strength3, this in turn limits the ability to absorb impact, through eccentric contraction and is borne out by the reduced knee flexion angles and increased early peak loading found during functional tasks, which is associated with increased patellofemoral joint loading14. So the local tissue homeostasis is significantly disrupted by both the presence of oedema and the altered loading environment.

The question then becomes how to reverse this downward spiral of quadriceps dysfunction, aberrant movement and perpetuation of the bony oedema. The most obvious place to break into this circle would appear to be at the beginning! Patients with greater quadriceps strength pre-op have superior function and at a variety of stages post op15, this also fits with what is known in PFP were those patients even with PFP who have stronger quadriceps have less pain and greater function(17). So first thing would be have the patients engage in a pre-operative strengthening programme (we’ll get onto defining what that means shortly) and very early post op starting strengthening and improving quadriceps function. Wang et al16 reported greater than 80% recovery of quads strength after ACLR is associated with less severe patellar cartilage damage, so it all appears to make sense. So why isn’t it happening already? The improvement in quadriceps function and reduction patellofemoral joint vulnerability are almost certainly impacted upon by two factors, first is a poor understanding of what strengthening exercises are and the second is the open kinetic chain exercise issue.

Though it might appear obvious, in order to strengthen muscles a suitable amount of external load needs to be lifted. The majority of patients undertake (body) weight bearing exercises (closed chain exercises such as squatting and step ups) which involve just moving their own body weight or small (relative to their body weight) additional external loads; they then do multiple repetitions of this. Plainly lifting a load for a large number of repetitions will not develop strength, this requires high loads which only can be lifted for a small number of repetitions. The best way to do this for quadriceps would be to do loaded open chain knee extension exercises; this takes us to the second problem the open chain exercise issue.

The open kinetic chain quadriceps exercise issue comes from the belief that quadriceps contraction creates anterior tibial translation and so could stress the ACL graft, there is no doubt that quadriceps contraction can create translation, but is it sufficient to actually create damage? If a maximal isometric quadriceps contraction is carried out at 20 degrees knee flexion (point of greatest potential translation), the level of translation load recorded is significantly less than that found undertaking a maximal lachmans test at the same angle(18). The amount of potential for anterior translation is angle specific with no translation occurring on quadriceps contraction at angles greater than 60 degrees knee flexion(19). In studies were open chain quadriceps exercises have been added to a rehabilitation programme, there was no significant differences in laxity across groups(20).

So to conclude, there is a strong likelihood that quadriceps dysfunction is related to PFP in the ACLR patient. In order to mitigate the risk, the patient needs to engage in a: preoperative training programme; early in the post op phase start to activate (overcome inhibition) and then strengthen the quadriceps. As part of this restricted range open chain knee extension exercises are likely to be a good option, alongside more traditional neuromuscular control exercises aimed at improving limb alignment and general whole limb work capacity.